Special-case some error messages about Sized

The error messages still aren’t as good as they were before DST, but they better
describe the actual problem, not mentioning `Sized` at all (because that bound
is normally implied, not explicitly stated).

Closes #17567.
Closes #18040.
Closes #18159.

Author: ftxqxd

src/librustc/middle/typeck/check/vtable.rs

@@ -303,15 +303,25 @@ pub fn maybe_report_ambiguity(fcx: &FnCtxt, obligation: &Obligation) {
         // has_errors() to be sure that compilation isn't happening
         // anyway. In that case, why inundate the user.
         if !fcx.tcx().sess.has_errors() {
-            fcx.tcx().sess.span_err(
-                obligation.cause.span,
-                format!(
-                    "unable to infer enough type information to \
-                     locate the impl of the trait `{}` for \
-                     the type `{}`; type annotations required",
-                    trait_ref.user_string(fcx.tcx()),
-                    self_ty.user_string(fcx.tcx())).as_slice());
-            note_obligation_cause(fcx, obligation);
+            if fcx.ccx.tcx.lang_items.sized_trait()
+                  .map_or(false, |sized_id| sized_id == trait_ref.def_id) {
+                fcx.tcx().sess.span_err(
+                    obligation.cause.span,
+                    format!(
+                        "unable to infer enough type information about `{}`; type annotations \
+                         required",
+                        self_ty.user_string(fcx.tcx())).as_slice());
+            } else {
+                fcx.tcx().sess.span_err(
+                    obligation.cause.span,
+                    format!(
+                        "unable to infer enough type information to \
+                         locate the impl of the trait `{}` for \
+                         the type `{}`; type annotations required",
+                        trait_ref.user_string(fcx.tcx()),
+                        self_ty.user_string(fcx.tcx())).as_slice());
+                note_obligation_cause(fcx, obligation);
+            }
         }
     } else if !fcx.tcx().sess.has_errors() {
          // Ambiguity. Coherence should have reported an error.

src/test/compile-fail/issue-16562.rs

@@ -16,8 +16,7 @@ struct Col<D, C> {
 }
 
 impl<T, M: MatrixShape> Collection for Col<M, uint> {
-//~^ ERROR unable to infer enough type information to locate the impl of the trait
-//~^^ NOTE the trait `core::kinds::Sized` must be implemented because it is required by
+//~^ ERROR unable to infer enough type information
     fn len(&self) -> uint {
         unimplemented!()
     }

src/test/compile-fail/issue-17551.rs

@@ -13,6 +13,6 @@
 struct B<T>;
 
 fn main() {
-    let foo = B; //~ ERROR unable to infer enough type information to locate the impl of the trait
+    let foo = B; //~ ERROR unable to infer enough type information
     let closure = |:| foo;
 }

src/test/compile-fail/issue-18159.rs

@@ -0,0 +1,13 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    let x; //~ ERROR unable to infer enough type information
+}